It is known in the field that a high-porosity porous body can be obtained by stretching a molded article produced by paste extrusion molding polytetrafluoroethylene fine powder. This polytetrafluoroethylene porous body consists of nodes (knots) and fibrils (fibers) and allows gas such as water vapor to pass therethrough, but does not allow water drops to pass therethrough owing to strong water repellency of polytetrafluoroethylene. This stretched porous body can be used as a sealing material without being sintered, or can be used in clothes or separation membranes after being sintered and formed into a tough, continuously stretched sheet or tube.
In particular, biaxially stretched porous membranes (biaxially stretched membranes) have been conventionally used in a wide variety of fields such as microfiltration membranes for gas and liquid (including liquid chemical), materials for covering electric wires, and breather valves.
Biaxially stretched polytetrafluoroethylene membranes are usually thin (100 μm or smaller, in general), and are likely to be broken during stretching steps, a taking-up step after the stretching, and post-steps such as lamination. Such membranes are also likely to be broken when used in clothes or separation membranes. Thus, these biaxially stretched membranes have problems in durability and reliability.
In order to produce a biaxially stretched polytetrafluoroethylene film having high strength, the following production methods are proposed.
For example, Patent Literature 1 and Patent Literature 2 disclose a manufacturing method for a porous film including stretching an paste extrudate containing an extrusion aid in a transverse direction, drying the aid, stretching the workpiece in the extrusion direction (machine direction) at least once, and further stretching the workpiece in the transverse direction.
Patent Literature 3 discloses a manufacturing method for a porous film including biaxially stretching a semi-sintered PTFE in the machine direction and then in the transverse direction, and heat-setting the biaxially stretched PTFE at a temperature not lower than the melting point of the sintered PTFE.
PTFE fine powder that can provide a highly strong porous body is also proposed.
For example, Patent Literature documents 4 and 5 disclose a high molecular weight tetrafluoroethylene homopolymer having a specific breaking strength.
Patent Literature documents 6 to 8 disclose a polytetrafluoroethylene aqueous dispersion obtained by polymerization in the presence of a specific emulsifier.
Patent Literature documents 9 to 11 disclose a tetrafluoroethylene-based copolymer modified with a perfluoroalkyl ethylene (PFAE).
Patent Literature 12 discloses non-melt-fabricable polytetrafluoroethylene fine powder for molding a stretched article and obtained by polymerizing tetrafluoroethylene and perfluoro(methyl vinyl ether).